The recent high performance and compactness of electronic devices such as mobile phones and IC cards require that semiconductor devices be further integrated, have smaller footprints, and have lower interconnection resistance due to a reduction in interconnection distance. To achieve this, a layered structure, i.e., a stacked structure, in which semiconductor devices are vertically stacked, has been studied.
One exemplary method for producing such a stacked structure includes, for example, the steps of bonding a silicon wafer having semiconductor devices formed thereon to a support substrate with an adhesive or the like for temporary bonding, and then thinning a rear surface of the silicon wafer; providing through-holes in the rear surface by the TSV (Through-Silicon Via) technique using a technique such as anisotropic dry etching; filling the through-holes with a conductive material such as copper to form electrodes through the rear surface; forming an insulating film on the rear surface of the silicon wafer having the electrodes formed therethrough; and electrically joining the resulting silicon wafer to another chip or silicon wafer having semiconductor devices formed thereon.
In these steps, the insulating film formed on the rear surface of the silicon wafer is required to have characteristics such as electrical insulating properties for preventing current leakage or migration of the conductive material between electrodes, solvent resistance, and heat resistance in the step of joining the electrodes. The insulating film is also required to exhibit these characteristics at a low temperature, for example, 250° C. or lower, from the viewpoint of the heat resistance of the component.
Known insulating films include insulating films formed by spin coating using polyimides, polybenzoxazoles, aromatic polyethers, and the like. In particular, an insulating film with photosensitivity can be readily formed by applying patterning through irradiation with active light, development, imidization by heating, and the like. Thus, compared to a non-photosensitive insulating film, such a photosensitive insulating film is characterized by a significantly shortened process. On the other hand, the development step of the photosensitive insulating film involves the use of a large amount of an organic solvent as the developing solution. Out of the recently growing concern for environmental issues, there is a demand for measures to eliminate the organic solvent. To meet this demand, a photosensitive resin material has been recently disclosed that it is obtained using a photosensitive polyimide precursor or a photosensitive polybenzoxazole precursor developable with an aqueous alkaline solution, as with photoresists (for example, Patent Document 1).
To exhibit good electrical insulating properties, however, such a photosensitive resin composition obtained using a photosensitive polyimide precursor or a photosensitive polybenzoxazole precursor needs to be baked for a long time at a high temperature, for example, about 350 to 400° C., so that the imidization by cyclodehydration proceeds completely. Thus, the use of the photosensitive resin composition at a low temperature, i.e., about 250° C., has been difficult. To allow the imidization to proceed at a lower temperature, a photosensitive resin composition has been disclosed that it is obtained using a polyimide or polybenzoxazole having an aliphatic chain structure (for example, Patent Document 2). The cyclodehydration reaction can be performed at a lower temperature by adopting the flexible aliphatic structure as the main chain structure, as described in Patent Document 2. It is, however, still difficult for the imidization to proceed completely, because the flexibility of the main chain skeleton decreases as the cyclodehydration reaction proceeds.
Photosensitive resin compositions obtained using aromatic polyethers developable with aqueous alkaline solutions have also been disclosed (for example, Patent Documents 3 and 4). Aromatic polyethers are superior to polyimides and polybenzoxazoles in that they do not need to be baked at high temperature to exhibit electrical insulating properties. The aromatic polyether described in Patent Document 3, however, has an acidic group such as a carboxylic acid to allow dissolution in an aqueous alkaline solution, and the acidic group is contained in the resin even after pattern formation. Thus, the aromatic polyether suffers from insufficient electrical insulating properties. The aromatic polyether described in Patent Document 4 has an amic acid structure as a side chain, and the imidization by cyclodehydration reaction is not affected by the main chain skeleton, which allows the imidization to proceed at low temperature. The aromatic polyether also has high electrical insulating properties. However, because a high temperature is required for the synthesis of the aromatic polyether, the hydrolysis or imidization of amic acid proceeds during the synthesis, which makes it difficult for the aromatic polyether to exhibit high solubility in an aqueous alkaline solution.